Network devices may be used to perform actions on the packets of data flowing through them. The actions performed may vary with the kind of data traffic flowing through the device and on the policies set up at the device. For example, the device can have a policy for marking all traffic from a particular subnet with high priority, send specific flows through a particular MPLS (Multi-protocol Label Switch) tunnel or VPN (Virtual Private Network), or collect per-flow statistics for accounting/billing purposes.
Packet classifiers are used to determine the policy applied by the network device. The policies applied to the packet may be determined, for example, from a search of the fields in the packet header. The packet classifiers (also known as “filters”) for the policies may be searched for matches with the packet header field each time a packet arrives at the device.